The invention proceeds from a hydraulic control arrangement which is used for a mobile working machine, especially for a wheel loader.
It is known from DE 39 09 205 C1 to make use for the purpose of damping the pitching vibrations of wheel loaders which occur, especially, with a full loading shovel and at a high driving speed of a damping system which is a component of the hydraulic control arrangement of the wheel loader. For the purpose of vibration damping, the generally two hydraulic lifting cylinders for raising and lowering the loading shovels can be connected via a shutoff valve to a hydraulic accumulator which can be charged by a hydraulic pump via a filling line which branches off from the pump line upstream of the directional control valve block. The shutoff valve, which is arranged between the hydraulic accumulator and the lifting cylinders, is closed as long as the loading shovel is working, and can be opened by the driver or automatically as soon as pitching vibrations occur during driving, or as soon as the driving speed is above a specific value, for example above 6 km/h.
As a consequence of the fact that the filling line branches off upstream of the directional valve control block, the hydraulic accumulator is charged not only upon actuation of the directional control valve assigned to the lifting cylinders, but upon actuation of any directional control valve which leads to a buildup of pressure in the pump line. For example, the actuation of a steering valve belonging to a hydraulic steering system of the working machine can also lead to an inflow of pressure fluid to the hydraulic accumulator. If the shutoff valve is then also open, an uncontrolled movement of the lifting cylinders can take place.
Another damping system against pitching vibrations, which is likewise part of the hydraulic control arrangement of a working machine, is known from DE 41 29 509 C2. In this case, the filling line branches off from a working line which runs between the lifting cylinders and the directional control valve assigned thereto. The shutoff valve arranged in the filling line is pressure-controlled and can be opened by the load pressure, prevailing in the working line, of the lifting cylinders against the accumulator pressure, which can be applied to a rear control chamber on the valve element of the shutoff valve, and against the force of a weak compression spring. The accumulator pressure is thus in each case only slightly lower than the highest load pressure of the lifting cylinders which occurs during a working cycle. In order to damp the pitching vibrations, the rear control chamber of the shutoff valve is unloaded via a pilot valve to the tank, with the result that the shutoff valve can be opened and pressure fluid can be pushed back and forth freely between the hydraulic accumulator and the lifting cylinders.
It is also known, in a hydraulic control arrangement with a hydraulic cylinder and with a directional control valve by means of which the pressure-fluid channels between a pressure chamber of the hydraulic cylinder, a pressure-fluid source and a tank can be controlled, to provide a pilot-operated check valve by means of which the pressure chamber is blocked off in a largely leak-free fashion when the directional control valve is in its neutral position. When pressure fluid flows to the pressure chamber, the check valve opens in its direction of flow. To allow pressure fluid to flow out of the pressure chamber, it must be released, and this can be accomplished by a second working line leading to a second pressure chamber of the hydraulic cylinder.
A line-rupture protection valve, which is generally seated directly on the hydraulic cylinder, can also be connected upstream of a pressure chamber of a hydraulic cylinder. A line-rupture protection valve is configured in such a way that pressure fluid can flow readily to the pressure chamber via a check valve opening toward the pressure chamber. To allow pressure fluid to flow out of the pressure chamber, a bypass to the check valve is opened to a greater or lesser extent. As disclosed, for example, in DE 32 39 930 C2, a control piston is adjusted in a proportional manner for this purpose. In the case of the line-rupture protection valve in the above-mentioned publication, it can for this purpose have applied to it the same precontrol pressure with which the directional control valve is actuated in one of its directions of motion.
It is the object of the invention further to develop a hydraulic control arrangement having the features from the precharacterizing clause of claim 1 in such a way that the pressure chamber of a hydraulic cylinder can be blocked off but that the connection, required for damping pitching vibrations, to a hydraulic accumulator can nevertheless be established.
This object is achieved, in accordance with the invention wherein there is a pilot-operated check value which is connected upstream of the pressure chamber of the hydraulic cylinder and opens toward it and can be controlled to open when the directional control valve is actuated to allow pressure fluid to flow off from the pressure chamber to the tank and when the control valve is actuated to establish a fluid connection between the pressure chamber and the hydraulic accumulator. In this way, the pressure chamber at the hydraulic cylinder and the hydraulic accumulator can be connected to one another even if a branch line leading to the hydraulic accumulator and located between the directional control valve and the check valve is connected to a working line leading from the directional control valve to the hydraulic cylinder, thus allowing the check valve to perform its blocking function even as regards this branch line. The term pilot-operated check valve is taken, very generally, to include a bypassable check valve such as that present, for example, in the case of line-rupture protection valves.
According to a feature of the invention there are three control lines, the third control line, to which release means for the shutoff valve are connected, being connected to a first control line via switch-over means when the directional control valve is actuated and being connected to a second control line when the control valve is actuated. In principle, electrical control lines, electrical control signals and electrical switch-over means are also conceivable here.
Very often, however, the shutoff valve is hydraulically releasable. An advantageous refinement concerns a hydraulically releasable shutoff valve.
If the directional control valve is hydraulically actuable, there is a precontrol oil circuit with a precontrol oil source from which the control oil for releasing the shutoff valve is also expediently taken. For the hydraulic actuation of the directional control valve, use is generally made of precontrol valves which operate on the basis of pressure-reducing valves and which have an inflow port, an outflow port and an outlet connected to a control chamber at the directional control valve. A constant maximum precontrol pressure is present in the inflow port. A pressure lower than the maximum precontrol pressure is set in the outlet depending on the adjustment of a pressure-reducing valve, and this pressure is applied to the directional control valve. The directional control valve is adjusted proportionally to different extents depending on the level of the pressure at the outlet of a pressure-reducing valve. It is expedient if the second control line is connected to that part of the precontrol oil circuit in which the maximum precontrol pressure prevails.
A precontrol oil circuit is not necessary if the control chamber of the shutoff valve can be connected to the hydraulic accumulator via the second control line. In operation, the hydraulic accumulator is namely charged up to the load pressure, and especially is always charged up to the highest load pressure which has occurred in the pressure chamber of the hydraulic cylinder until a maximum pressure is achieved, thus ensuring that, when the control chamber of the shutoff valve is connected to the hydraulic accumulator, a sufficient pressure to open the shutoff valve is available.
To make very certain that no opening pressure builds up in the control chamber of the shutoff valve when the directional control valve is unactuated and the control valve is unactuated, a switch-over valve is provided via which, in a first control position which it assumes when the control valve is unactuated, the second control line is connected to the tank. It is then possible to use a simple hydraulic shuttle valve as a switch-over valve to connect the third control line to the first control line or the second control line so as to select maximum pressure.